1. Field of the Invention
The present invention relates to a distributed feedback semiconductor laser used for optical fiber communications or the like.
2. Description of the Related Art
As is well-known, in optical fiber communications, an optical signal generated by modulating a light emitting element such as a semiconductor laser is transmitted through an optical fiber. A longitudinal single mode semiconductor laser is used for long-distance transmission in order to prevent the semiconductor laser from being deteriorated by dispersion of the optical signal in the optical fiber. A longitudinal single mode semiconductor laser having good low-noise characteristics is used in an analog multichannel image transmission such as cable television.
The longitudinal single mode semiconductor laser is embodied by a distributed feedback (referred to as DFB hereinafter) semiconductor laser. As shown in FIG. 1, the DFB semiconductor laser comprises a diffraction grating 22 formed on a semiconductor substrate 21, a wave guide layer 23, an active layer 24, a clad layer 25, and a contact layer 26. A bias current If is supplied to electrodes 27 and 28 between which the substrate 21, grating 22, and layers 23-26 are interposed, to inject the current into the active layer 24. The active layer 24 is thus oscillated and, as shown in FIG. 2, a single wavelength .lambda., which depends upon a pitch P of the diffraction grating 22, and composition and dimension of the wave guide layer 23 and active layer 24, can be obtained.
According to the DFB semiconductor laser having the above structure, photons and electrons in the wave guide layer 23 and active layer 24 interact with each other only in the single wavelength mode. Therefore, the distribution of an internal electric field is strongly influenced by conditions such as pitch P of the diffraction grating 22, refractive index of the wave guide layer 23, and dimension and precision of the active layer 24 and wave guide layer 23. If the conditions are not optimal, an extremely nonlinear output (kink) characteristic or a multi-wavelength oscillation appears in accordance with the density of the injected current. In FIG. 3, samples #1 and #4 show normal characteristics, but samples #2 and #3 show kink characteristics. As a result, there occurs problems of decrease in yield and reliability. Although the longitudinal single mode semiconductor laser having the low-noise characteristic and the low-distortion characteristic is the most suitable for the analog multichannel video transmission, it has the problem of kink characteristic.
FIG. 4 is a cross sectional view showing a structure of a Fabry-Perot (referred to as FP hereinafter) semiconductor laser. As shown in FIG. 4, an active layer 34, a clad layer 35, and a contact layer 36 are formed in sequence on a substrate 31, and these layers substrate are interposed between electrodes 37 and 38. Mirror reflecting surfaces 39a and 39b are formed on both facets of the FP semiconductor laser. Reference character L denotes length of the resonator. FIG. 5 shows power spectrum of the FP semiconductor laser. Since the FP semiconductor laser has no diffraction grating and photons and electrons interact with each other in a plurality of modes, the FP semiconductor laser is excellent in the linearity in light emitting characteristic and thus suitable for analog modulation. However, the interaction of electrons and photons in a plurality of modes causes mode partition noises, and the relative intensity of noise (RIN) of the FP semiconductor laser is greater than that of the longitudinal single mode semiconductor laser by about 20 dB. Thus FP semiconductor laser cannot be used in analog applications.
As described above, in the conventional longitudinal single mode semiconductor laser having low-noise characteristic and low-distortion characteristic, photons and electrons in the active layer and wave guide layer interact with each other only in the single wavelength mode, so that the distribution of the internal electric field is strongly influenced by the pitch of diffraction grating and the dimension and precision of the active layer and wave guide layer.